We propose to refine and exploit powerful new genetically encoded labeling systems to visualize proteins by electron microscopy (EM), correlated with light microscopy (LM). EM is one of the most powerful techniques to see cell structures below optical resolution, but has suffered from lack of generally applicable genetically encoded labels until our recent development of miniSOG, a small flavoprotein that will do for EM what Green Fluorescent Protein did for LM. We aim to quantify the sensitivity and spatial resolution of miniSOG and to extend its applicability to low-temperature methods for sample preparation and imaging. Alternative genetically encoded labels will be developed and characterized to allow for two or more proteins of interest to be distinguished in a single EM image by electron energy loss spectroscopy of reaction deposits from oxidation of diaminobenzidine conjugated to different lanthanides. We have also developed reporters, based on the drug- controllable cis-acting protease from hepatitis C viral protease, to distinguish between old and newly synthesized copies of a genetically specified protein of interest. These fusion tags are visible by correlated LM and EM and will be applied to plasticity and disease-related synaptic proteins to reveal their localized appearance and turnover, initially in culture bt eventually in intact mammalian brain. Viral and Cre/lox modular targeting vectors will be created to make cell-type-selective expression of the above EM/LM tags robust and widely applicable to systems as complex as mouse models of disease and learning. We have chosen cell types and proteins important in liver fibrosis and synaptic plasticity as test cases because these biological processes are diverse, engage outstanding local collaborators, and have great biomedical importance. With collaborators we will investigate how hepatocytes, hepatic stellate cells, and endothelial cells change ultrastructural morphology and location of key proteins during fibrogenic injury. Another collaboration will focus on activity-induced changes in morphology and adhesion molecules at synapses in the amygdala during fear conditioning and memory consolidation. Such extension of these new tools into transgenic animals will make it possible to relate ultrastructural location and metabolic turnover of genetically specified proteins to whole- animal behavior and disease.